In a forward converter topology, it is known to drive a synchronous rectifier and a free-wheeling MOSFET directly from a secondary output of a transformer. However, in low output voltages or high density applications, designers prefer a circuit having better efficiency than such configurations typically provide.
Driving the synchronous rectifier and free-wheeling MOSFET directly from the sole secondary winding of the transformer may be acceptable in a DC-DC converter providing a high output voltage, but a DC-DC converter with a low output voltage generally requires a different driving scheme to improve efficiency. An example of a different driving scheme includes using a transformer having a main secondary winding and at least one auxiliary secondary winding that has a higher turns ratio than a main secondary winding. The auxiliary winding having the higher turns ratio is used to provide a drive voltage for the synchronous rectifier and the free-wheeling MOSFET. However, using a transformer with a turns ratio greater than 1:1 is undesirable in a DC-DC converter design that uses a planar transformer and/or is of compact physical dimensions such as an industry standard ⅛ or 1/16 brick form factor.
Generating a bias voltage is another issue that may arise in a design using a planar transformer and/or a compact design. Again, with a large transformer having a high secondary output voltage, the bias voltage can be tapped from the main secondary transformer winding. In a low output voltage application, however, another solution is needed.